Fish farming entails growing fish in a controlled environment for commercial purposes. Installations in use for this purpose generally include one or more containments for holding fish in water where they can be fed and grow and from which they can be harvested. The containments may include tanks or ponds. Some containments, typically tanks, may be enclosed within buildings.
In order to promote healthy fish growth, the water in the tank or pond must be kept generally clean and suitably oxygenated. Moreover, in cases where water is ejected from fish farming installations into streams or waste water treatment facilities, there is also a concern regarding the levels of biological contamination of the effluent water. Waste products from the fish and uneaten fish food are major sources of biological contamination. Fish consume oxygen to support metabolism, and the decomposition waste and food in the water increases oxygen demand.
Fish farming installations may be of the flow-through type or the closed system type. Flow through installations benefit from sufficient water to allow no recirculation of water back into the containment. For some fish species, flow-through fish culture is desirable. In these types of installations, there is a concern that the water flowing from the fish farming installation does not cause an unacceptable pollution problem downstream due to biological contamination and oxygen depletion.
More common, however, are closed loop type installations, otherwise known as re-circulating aquaculture systems. In closed loop installations, the water is recirculated, often as frequently as one tank volume per hour. Many commercially-important warm-water species thrive in such installations. During re-circulation, the water is filtered to remove biological contaminants and re-oxygenated by aeration or the injection of oxygen gas. Generally, injection of oxygen gas is preferable over aeration because of elevated nitrogen levels that can result when attempting to diffuse sufficient oxygen into the water by using air.
Among the challenges included relative to diffusing oxygen into the water is the problem of oxygen bubbling out of solution to the atmosphere and thus being unavailable to support biological processes in the water, including, of course, fish metabolism as well as degradation of biological contaminants in the water.